Tires, high performance tires, in particular, have been required to have good maneuverability, as the capability of cars and highway conditions improve. Wear strength, breaking properties, and grip properties, which may be indicated by wear strength, stability at high speed, accelerating capability and breaking capability are the properties most in demand. It has been desired to improve these properties, and, further, those which give priorities to steering properties, good high speed stability and good durability at high speed.
To provide these rubber compositions, the relationship between wet skid resistance or dry grip properties and viscoelasticity of rubber compositions has been studied. It has been found that grip properties can be improved by increasing hysteresis loss value, that is, by increasing tan.delta. at 50.degree.to 70.degree. C. and by compounding large quantities of extending oil. It has also been found that improvement in wear strength and durability at high speed can be effected by improving tensile strength and elongation at break, both at high temperature, i.e., around 100.degree. C. In this regard, it is known that these properties can be improved by using an existing rubbery polymer or a novel polymer, i.e., by adding such polymers to the tire tread compositions, to make a novel composition.
As examples of the former method, a butyl rubber, (see JPA62-143945), a polynorbornene (see JP-A-62-143945 or see JP-A-2-142838), a polyisoprene (see JP-A-63-132949), or a cumarone-indene resin (see JP-A-62-1735) have been added to a butadiene-styrene polymer (SBR). As examples of the latter method, an SBR has been modified with a diphenylmethyl alcohol iderivative (see JP-A-60-61314), a diblock butadiene-styrene polymer has been used as a rubbery component (see JP-A-1-131258), an SBR has been polymerized using an organolithium compound and certain organic compounds such as potassium butoxide, as a rubbery component (see JP-B-44-20463), and an SBR has been a polymerized using a potassium salt as a randomizer as rubbery component (see JP-A-3-239737).
But, since a lot of extending oil has been used to improve grip properties, processability was deteriorated. So it is now required to improve both these properties and processability at the same time. The conventional methods, even those with controlled compounding, could not provide sufficiently good properties. Moreover, little effort has been put into improvement of processability.